I did a back of the napkin calculation the other day to see if a botnet could reasonably reach the 50% computing power required to overwrite transactions. The largest known botnet (BredoLab @ 30,000,000 bots) with modest hardware could have potentially done it. This computational power is a multiple of the combined computing power of the top 100 supercomputers in existence today.

Of course, botnets are frequently on older machines, so the actual computational power of BredoLab probably wouldn't have been enough. Also, as popularity increases, this will become less reasonable.